In packet switched data transmission, the data to be transmitted in a network is divided into small data units, called packets. These packets that comprise the address information of a recipient, are transmitted from a sender to the recipient by routing their path in the network on the basis of the recipient's address. In packet switched data transmission, the same radio resources can be divided among multiple users, as necessary.
GPRS is the GSM (Global System for Mobile communications) network's packet switched data transmission service that complements the existing services, such as conventional circuit switched data transmission and the short message service (SMS). In conventional circuit switched data transmission, the allocation of radio resources between a wireless terminal, such as a mobile station or a computer terminal, and a base station subsystem (BSS) is typically carried out by reserving a so-called physical (radio) channel for the duration of the call, where a physical channel means a specific time slot of a transfer frame on a given frequency band. GPRS, which is defined in general in the GSM recommendation 03.60, enables the dynamic allocation of physical channels for data transmission. In other words, a physical channel is being reserved for a particular MS-BSS link only when there is data to be transmitted. Thus, the unnecessary reservation of radio resources when there is no data to be transmitted is avoided.
GPRS is intended to operate in conjunction with conventional GSM circuit switched transmission to efficiently use the air interface for both data and voice communications. GPRS therefore uses the basic channel structure defined for GSM. In GSM, a given frequency band is divided in the time domain into a succession of frames, known as TDMA (Time Division Multiple Access) frames. The length of a TDMA frame is 4.615 ms. Each TDMA frame is in turn divided into eight consecutive time slots of equal duration. In the conventional circuit switched transmission mode, when a call is initiated, a physical channel is defined for that call by reserving a given time slot (1-8) in each of a succession of TDMA frames. Physical channels are similarly defined for conveying various signalling data in a network.
With taking GPRS into use in the GSM system, radio resources for data transmission are reserved by dynamically assigning physical channels either for circuit switched transmission mode or the packet switched transmission mode. When the network requirements for circuit switched transmission mode are high, a large number of time slots may be allocated to that mode. The GSM network service that offers multiple time slots for the use of the same circuit switched connection in the same TDMA frame, is called a HSCSD (high speed circuit switched data) service. On the other hand, when demand for GPRS transmission mode is high, a large number of time slots may be allocated to that transmission mode. In addition, a high-speed packet switched transmission channel may be provided by assigning two or more slots in each of a succession of TDMA frames to a single wireless terminal. A series of four consecutive time slots on a physical channel is known as one data block, and it represents the shortest packet switched data transmission unit on a physical channel.
FIG. 1 shows telecommunication network connections in a packet switched GPRS service. The main element of the network's infrastructure for GPRS services is a GPRS support node, which in packet switched data transmission corresponds to the GSM network's mobile switching center MSC, known in connection with circuit switched data transmission. GPRS support nodes are divided into serving GPRS support nodes SGSN and gateway GPRS support nodes GGSN. SGSN is a support node that transmits data packets to a wireless terminal MS (mobile station) and receives the data packets sent by the MS, through a base station sybsystem BSS that is formed of base transceiver stations BTS and base station controllers BSC. In this description, by a wireless terminal MS is meant all data terminal equipment that communicate over a specific radio interface. Thus, also a computer terminal that communicates through a mobile station coupled thereto is called a wireless terminal. SGSN also maintains together with GPRS registers (not shown in the figure) the location data of wireless terminals that move in its service area. Physically, SGSN is typically implemented as a separate network element. GGSN that communicates with SGSN implements the connection and co-operation with other networks. These other networks may be, among others, the GPRS network of some other operator or some private network, Internet network/public switched packet data network PSPDN or X.25 network.
The GPRS radio interface between a BTS and an MS is called the Um interface. Said Um interface for GSM Phase 2+ (GSM 03.64) can be modelled as a hierarchy of logical layers with specific functions. As is shown in FIG. 2, a wireless terminal (mobile station, MS) and a network have identical layers, which communicate via the Um interface between the mobile terminal and the network. It should be understood that the model shown in FIG. 2 does not necessarily represent the hardware contained by the mobile station and the network, but it rather illustrates the flow and processing of data through the system. Each layer modifies data received from the neighbouring layer. Typically, the received data passes through the logical layers from the bottom to the top layer and the data to be transmitted passes from the top to the bottom layer.
Under the application layer, in the top logical layer shown in FIG. 2, the MS has multiple packet data protocol (PDP) units. Some of these PDP units use point-to-point protocols (PTP) adapted for sending packet data from one wireless terminal to another or from a wireless terminal to a fixed terminal. Examples of PTP protocols are IP (Internet Protocol) and X.25, which are capable of establishing an interface with applications of the application layer. In the network, similar protocols with which protocols of the top layer of the mobile station communicate are typically located in a gateway GPRS support node (GGSN).
The top layer units use a subnetwork dependent convergence protocol (SNDCP, GSM 04.65), one task of which is to compress and divide, as well as compile data into SNDCP Packet Data Units. In the network, a similar SNDCP layer is typically located in a serving GPRS support node (SGSN).
The logical link control (LLC, GSM 04.64) layer provides a reliable encrypted logical connection between the MS and the SGSN. LLC frames, formed by the LLC layer, are used for conveying SNDCP packet data units (or other GPRS endpoint protocol units) over the radio interface.
The RCL/MAC (radio link control/medium access control, GSM 04.60) layer provides services for transmitting information over the physical layers of the GPRS radio interface between the MS and a base station subsystem. The RLC/MAC layer comprises two different functions: RLC function comprises, among other things, procedures for segmenting LLC layer data blocks and reassembling them into RLC data blocks. The RLC function also comprises procedures for re-transmitting unsuccessfully delivered RLC blocks. The MAC function acts above the Phys. link layer and defines the procedures that enable radio resources to be allocated and divided among multiple users. The MAC function also arbitrates between wireless terminals that are trying to transmit data simultaneously, by providing collision avoidance, detection and recovery procedures. Physically, the network's RLC/MAC layer is typically located in a base station subsystem BSS, in a base station controller BSC, wherein it is typically implemented by a so-called packet control unit (PCU). It is also possible to place the PCU in a SGSN or a BTS.
The physical link layer provides a physical channel between the MS and the network. The physical RF (radio frequency) layer defines, among other things, the carrier frequencies and GSM radio channel structures, the modulation of the GSM channels and the transmitter and receiver characteristics.
When the MS has data to transmit, the RLC/MAC layer's MAC function allocates, from the network, the radio resources necessary for transmitting the data over the radio interface. Typically, in this case, a TBF (Temporary Block Flow, GSM 03.64) connection is being established, which is a temporary physical one-way connection between the MS and the network for transmitting data blocks on a physical channel over the radio path. Here, its temporary nature means that TBF is only maintained for the duration of data transmission.
There are two types of TBF connections: close ended TBF and open ended TBF. In a close ended TBF, the network allocates to the MS for the transmission of data blocks a pre-determined number of time slots in a succession of TDMA frames, depending on the number of data blocks to be transmitted. In an open ended TBF, the number of data blocks to be transmitted during the connection is typically not known to the network in advance. Therefore, in an open ended TBF, the network allocates time slots to the MS until the open ended TBF connection is released either by the network or the MS. The open ended TBF is released, for example, if the network detects that the MS has not transmitted data during a given number of frames. Unlike in the close ended TBF, in the open ended TBF, the duration of the connection is not known to the network in advance. Therefore, the network prefers to allocate closed TBF connections to the MS, because then it has better opportunities to efficiently divide radio resources among different users.
For allocating radio resources (for establishing a TBF connection), there are substantially two different alternatives (FIG. 3a-3b): 1-phase access and 2-phase access.
In 1-phase access (GSM 04.60), the MS sends the network a packet channel request. The packet channel request is in its encoded form eight or eleven bits long, depending on the network. Among other things, a so-called Multislot Class parameter is encoded by five bits in the packet channel request, which parameter indicates how many time slots the MS is able to use at a maximum, but due to the shortness of the message, not much other data can be supplied to the network therein. In response to the packet channel request, the network typically sends the MS a specific assignment message wherein the network assigns radio resources to the MS by typically establishing a close ended TBF connection for the MS. Typically, said assignment message is a packet uplink assignment message, wherein the network assigns the radio resources to the MS for an uplink radio transmission. Among other things, the time slots during which the MS can transmit appear from the message.
In 2-phase access, the MS sends the network two messages. First, the MS sends a packet channel request, wherein it only asks the network for radio resources for the transmission of a packet resource request. After again receiving a specific assignment message from the network, the MS sends the packet resource request, which is one radio block in length. In the packet resource request, the MS may send the network a lot of information (values of different parameters) on the basis of which the network decides on the allocation of the radio resources.
In the one-bit long RLC_MODE frame (GSM 04.60), the MS may propose a specific transfer mode for the TBF connection it requested. If the bit is 1, the MS is proposing the unacknowledged RLC mode. If the bit is 0, the MS is proposing the acknowledged RLC mode. In the acknowledged transfer mode, acknowledgements are used so as to find out the error-free delivery of the RLC data blocks. The acknowledged transfer mode also provides the RLC function an opportunity to retransmit unsuccessfully transferred data blocks.
In the sixteen-bit long RLC_OCTET_COUNT field, the MS may propose the TBF connection being established to be either a close ended TBF connection of specific duration or an open ended TBF connection. However, the network may ignore the MS's proposal and decide alone what type of TBF connection will be established.
In response to the packet resource request, the network again sends the MS a specific assignment message, wherein the network assigns radio resources to the MS establishing a close or open ended TBF connection for the MS.
GPRS was initially designed for non-realtime data services, such as e-mail services. However, the pressure for using GPRS in realtime services that require a short delay, such as in the transmission of speech (voice) and a video image, is continuously increasing. In the following, by the term realtime data transmission is meant explicitly data transmission for realtime services. For realtime services, the following three requirements are set on GPRS:                use of open ended TBF        short access delay        short transmission delay        
The use of an open ended TBF connection in realtime services is important in order to avoid, for example, constant breaks due to the establishment and release of close ended TBF connections of specific lengths, e.g. in speech transmission. However, the problem is that according to current GPRS specifications, the network can decide alone what type of TBF will be established. But, as has already been mentioned, said network prefers to allocate to the MS close ended TBF connections, because then it has better possibilities to efficiently divide radio resources between different users.
Achieving a short access delay is important because, for example, when transmitting speech the TBF connection will be cut off during a quiet period so that a new TBF connection must again be established when the silence ends. A short access delay is attained by using 1-phase access. But when using 1-phase access, obtaining an open ended TBF connection is not at all certain, because the network decides what type of TBF connection will be established at any given time.
Achieving a short transmission delay is a distinct requirement in connection with realtime services. A short transmission delay is attained by using the unacknowledged RLC mode. However, according to the current GPRS specification (GSM 04.60 version 6.4.0), the acknowledged RLC mode should be used when requesting a TBF connection with 1-phase access.